But what is a special counsel, and how will his investigation work? Here are answers to questions you might have.
Q: What is a special counsel?
A: A special counsel is a person appointed by the attorney general — or, in this case, the deputy attorney general, because Attorney General Jeff Sessions is recused from the Russia case — to oversee a criminal probe. Such an appointment is made when the Justice Department has a conflict of interest or it is otherwise in the "public interest" to do so. The special counsel oversees investigators and ultimately decides whether criminal charges are warranted. If so, the special counsel can prosecute the case.
Q: Does this mean Trump or his associates committed a crime?
A: Not necessarily. Mueller could bring charges, but he could also decline to do so. In a statement announcing the appointment, Rosenstein stressed that his decision was "not a finding that crimes have been committed or that any prosecution is warranted." He said that "based upon the unique circumstances, the public interest requires me to place this investigation under the authority of a person who exercises a degree of independence from the normal chain of command."
Q: What can the special counsel investigate?
A: In this case, Mueller was tasked with investigating "any links and/or coordination between the Russian government and individuals associated with the campaign of President Donald Trump," according to Rosenstein's order. He also has authority to investigate "any matters that arose or may directly arise from that investigation."
That would seem to include possible efforts to impede the investigation, including Trump's alleged request to shut down a probe related to former national security adviser Michael Flynn. The law also allows the special counsel to investigate efforts to impede his own work.
Q: Where do the special counsel's money and people come from?
A: Resources come from the Justice Department. Within 60 days, Mueller will have to come up with a proposed budget, which Rosenstein will have to review and approve.
Mueller can request that Justice assign employees to work for him, or he could ask that outsiders be brought in. It is possible — even likely, given the probe's sensitive counterintelligence nature — that Mueller would work with the current FBI agents on the case, though he would be the ultimate decider.
Q: To whom does the special counsel answer?
A: The special counsel is not subject to the "day-to-day supervision" of any Justice employee, but the buck does not necessarily stop with him.
The attorney general — or, in this case, Rosenstein — can "request that the Special Counsel provide an explanation for any investigative or prosecutorial step, and may after review conclude that the action is so inappropriate or unwarranted under established Departmental practices that it should not be pursued," according to the law on the topic. The attorney general, though, must "give great weight" to the special counsel's views, and, if he intervenes, must notify the chairman and ranking members of the congressional judiciary committees with an explanation for his actions.
Q: Is this the same thing as an independent counsel?
A: It's not, but it's close.
The independent counsel was a position established by a law that has expired, in part because of dissatisfaction over the independent investigations of Bill Clinton. Unlike the special counsel, the independent counsel was formally appointed by a panel of judges from the U.S. Court of Appeals for the District of Columbia Circuit, and those judges oversaw the independent counsel's work.
Q: What's the precedent for this?
A: Special counsels like this are rare, and there is only one good precedent. Attorney General Janet Reno named former senator John C. (Jack) Danforth to review the events surrounding the law enforcement assault on the Branch Davidian compound in Waco, Tex., in 1993.
Reno also appointed Robert B. Fiske Jr. to investigate the Whitewater scandal in the Clinton administration, though he was later replaced by an independent counsel.
In 2003, then-Deputy Attorney General James B. Comey appointed Patrick Fitzgerald as a sort-of special counsel to investigate whether George W. Bush administration officials leaked the name of a CIA operative, but Fitzgerald at the time was the U.S. attorney in Chicago, meaning he was not totally outside of Justice.
Q: Will the special counsel's work be public?
A: Not necessarily. The law says that at the end of the probe, the special counsel has to give to the attorney general a "confidential report" explaining his decision to prosecute or not. The attorney general "may determine" that the public release of the report would be in the public interest, but that is not required.